Hydraulic mounts are used in many situations where it is desired to isolate sources of vibration, or to protect sensitive equipment from shock and vibration. Examples include, but are not limited to: industrial equipment and machinery isolators; industrial robotics; building, bridge and ship isolators; military weapons systems; agricultural equipment; and construction equipment. Hydraulic mounts are also often used with vehicle powertrains to control movement of the powertrain in response to forces, such as reaction torque and vibration. The mounts serve a second function, that of isolating the engine from the body of the vehicle. A well-known type of hydraulic vibration damping mount generates damping in a predetermined frequency range of vibrations by pumping a hydraulic fluid through an orifice track of predetermined dimensions. The dimensions of the orifice track are typically such that the hydraulic fluid resonates at certain frequencies of input vibration, which maximizes the damping of the mount. At vibration frequencies above the track resonance the dynamic rate of the mount increases, reducing the isolative properties of the mount. Hydraulic mounts may also be provided with devices known as decouplers, which are disposed in a space formed within the mount orifice plate, for example, and allowed limited free travel within the space to “short circuit” the fluid from flowing through the orifice track, thus generating a low magnitude of dynamic stiffness necessary to provide isolation of certain vibrations. When the input vibration to the mount exceeds the allowable limit of the free motion of the decoupler, the hydraulic fluid flows through the orifice track, thereby generating the mount damping characteristics.
For optimum isolation, the dynamic rate of the mount at the input vibration or “disturbance” frequency should be as low as possible. Since the resonant frequency of the hydraulic damping mount is fixed by the dimensions of the orifice track, prior art mounts must be designed to cover as broad a range of vibration characteristics as possible, or to damp the most prevalent vibration frequencies, to provide effective damping. This necessarily results in a tradeoff or compromise in the performance of the mount. For example, a vehicle's powertrain exhibits varying vibration characteristics as the engine changes from an idle state, where the engine is operating at a low rate (typically measured in revolutions per minute or “RPM”), to an operating state, where the engine operates at a higher RPM. These changing input vibration frequencies are imposed upon the mount. However, due to the fixed physical properties of the mount, the mount's effectiveness at damping the vibration will be greater or lesser, depending upon the mismatch between the disturbance frequency and the resonant frequency of the mount. Accordingly, there is a need for a hydraulic damping mount that provides improved performance over a broader range of disturbance frequencies.